Integrated circuits, the key components in thousands of electronic and computer products, are interconnected networks of electrical components fabricated on a common foundation, or substrate. Fabricators typically use various techniques, such as layering, doping, masking, and etching, to build thousands and even millions of microscopic resistors, transistors, and other electrical components on a silicon substrate, known as a wafer. The components are then “wired,” or interconnected, with aluminum wires to define a specific electric circuit, such as a computer memory. The aluminum wires, normally embedded in insulation, are typically about one micron thick, or about 100 times thinner than a human hair.
As integrated circuits have become progressively smaller and more densely packed, the wiring connecting components has inevitably been spaced closer together. Unfortunately, this closer spacing has increased capacitance between wires. Increased capacitance not only causes cross talk—undesirable signal mixing between adjacent wires—but also wastes power and slows response of integrated circuits to electrical signals. Thus, fabricators are generally concerned with ways to reduce capacitance.
One way to reduce capacitance between the wires is to separate them with an insulation better than silicon dioxide, the most prevalent insulation material. Insulations are rated in terms of a dielectric constant, with lower dielectric constants giving rise to less capacitance than higher dielectric constants. Thus, to reduce capacitance, one can replace the typical silicon-dioxide insulation, which has a dielectric constant of about 4, with an insulator having a lower dielectric constant.
Air, which has a dielectric constant of about 1, is one such insulator. In fact, there are very few, if any, practical insulators with a lower dielectric constant. To use air as an insulator, fabricators have developed an interconnect structure called an air bridge—a conductive wire that extends through an air-filled space of an integrated circuit Most commonly, the microscopic wire bridges a space between two pillars that support its ends. Two air bridges can be placed side by side such that air separates their respective wires, thereby dramatically reducing capacitance between the two wires.
Unfortunately, conventional fabrication techniques are limited to making short air bridges, typically with unsupported, or free-span, lengths about 1 millimeter long (assuming a 500-nanometer thickness and a maximum allowable sag of 250 nanometers). The air bridges must be kept short because the typical aluminum alloy used to make air-bridge conductors is too supple and the conductors sag in the middle, sometimes forming short circuits with other conductors or even breaking. This aluminum alloy includes small amounts of copper and silicon to enhance its electromigration resistance, that is, its resistance to disintegration at high current levels, but nothing to promote its stiffness, or rigidity.
Theoretically, one can enhance rigidity of any given alloy by adding metals known for their rigidity to the alloy. However, most metals that would enhance rigidity of the typical aluminum alloy also substantially increase its density (mass per unit volume) or its electrical resistance, generally rendering the resulting wires too heavy or too electrically resistive to benefit air-bridge applications. For example, adding 25 weight-percent iron, a rigid metal, to the typical aluminum alloy would increase its rigidity about 4 percent but increase its density about 20 percent and its electrical resistance (per unit length) about 15 percent. Increased density makes wires heavier, more prone to sag, and thus less apt to improve air-bridge lengths, while increased resistance wastes power, slows down integrated circuits, and thus offsets the promised advantages of using longer air bridges.
Accordingly, to achieve longer, performance-enhancing air bridges, there remains a need for alloys which have not only better rigidity but also appropriately low electrical resistance and mass density.